Mass towers of older construction are designed so that the pulp is, at least when discharged, of low consistency i.e. below about 5%. The pulp stored in the tower either has a consistency which is sufficiently low so that the pulp can be discharged entirely by conventional means or, if initially at higher consistency, the pulp is diluted in the bottom part of the tower to a lower consistency so that, in either case, the pulp can be pumped by a conventional low consistency pump arranged at the side wall of the bottom part of the mass tower. Therefore, the tower has conventionally been constructed and arranged so that the bottom of the tower rests directly on the ground and the pumps have been installed on the same ground level. Such a structure is naturally advantageous as the entire weight of the tower and pulp contained therein is evenly distributed over the entire area of the tower bottom.
To convert the known "low consistency" towers to meet the demands of modern medium consistency technology, one must take into account that medium or high consistency pulp is generally discharged through the bottom of the respective tower. The term medium or high consistency pulp refers to pulp having a consistency above 6% and preferably up to 10% or higher. Therefore, it is required to raise the bottom of the known tower to enable the installation of a pump underneath the tower bottom. Accordingly, there have been attempts to raise the tower bottom of a conventional mass tower so that a new bottom can be attached e.g. by welding, to the walls of the tower leaving a space for the pump and other required auxiliary equipment between the raised tower bottom and the ground level. However, such structures have not proved to be reliable, as the original walls of the tower are relatively thin and have not been designed to bear the additional stress due to the weight of the pulp when the tower has been raised. In other words, stiffening and reinforcing members must be installed to support the weight. Additional problems are caused by changes in the shape of the tower bottom due to both temperature changes and changes in the weight of the tower, as the pulp level in the tower may change to a great extent as the tower may at times be completely empty or it may be completely filled.
A very commonly used structure of a mass tower is one built up of concrete bricks. In this structure it is very difficult to install a raised steel bottom and to shape the same in any desired way. Also the space under the new steel bottom cannot be efficiently utilized due to structural demands which also add greatly to the expense of any structural modification.